This is dependent on your goals and what your age cutoff is. Violins are one of the hardest musical instruments to become proficient at but kids still learn it and become adept. Programming languages, using the command line and other steep learning curve tools have the potential to offer large rewards for learning how to use them, even if they're hard to learn.

In terms of the specific pieces of software, I don't think Photoshop is so much easier to learn than GIMP and GIMP has the added benefit of being libre/free and gratis. There's also Krita [0].

[0] https://krita.org/en/